Incorporation of Activated Biomasses in Fe-N-C Catalysts for Oxygen Reduction Reaction with Enhanced Stability in Acidic Media

Fe-N-C materials are promising oxygen reduction reaction (ORR) catalysts for replacing expensive platinum-based catalysts (Pt/C) in proton exchange membrane fuel cells. However, they still show low volumetric activity and stability compared to Pt/C catalysts, with carbon corrosion being one of the main factors for the loss of active Fe-Nx sites. Within this study, phosphoric acid-activated rye straw and coconut shells are revealed as a promising matrix for Fe-N-x sites with advanced stability against electrochemical carbon corrosion (5000 cycles, 1.0-1.5 V-RHE, and 0.1 M HClO4) compared to a common Fe-N-C catalyst based on carbon black. Electrochemical characterization of the two biomass-based catalysts (Fe-N-CBio) shows on the one hand 50% higher stability in terms of mass activity as well as comparable activity and active site density but on the other hand a lower selectivity toward the four-electron ORR than the common Fe-N-C catalyst. Nitrite stripping experiments in acetate buffer as an electrolyte display a 1.5-fold stronger effect of carboxylic acid adsorption than on a common Fe-N-C catalyst, revealing differences to the electronic structure of the Fe-N-C-Bio catalyst. This difference is mainly attributed to the presence of phosphor species and higher amounts of nitrogen functionalities in the Fe-N-C-Bio catalysts. The presence of P is assumed to stabilize the carbon against carbon corrosion by inhibiting electron withdrawal from the C. This study points out the impact factors on Fe-N-C stability and further shows the promising application of activated biomasses in more stable and sustainable Fe-N-C catalysts for ORR.

Automated summary

This study reveals phosphoric acid-activated rye straw and coconut shells as promising matrices for stable Fe-N-x sites, leading to Fe-N-C catalysts with higher stability against carbon corrosion. However, these biomass-based catalysts show lower selectivity towards the four-electron ORR compared to common Fe-N-C catalysts.